Selective etching method, optoelectronic device and its fabricating method

ABSTRACT

When a ZnSe, ZnTe or ZnSSe or Zn 1-a  Mg a  S b  Se 1-b  (0&lt;a&lt;1, 0&lt;b&lt;1) layer provided on a Zn 1-x  Mg x  S y  Se 1-y  (0&lt;x&lt;1, 0&lt;y&lt;1, a&lt;x) layer is selectively etched by a dry etching method such as an RIE method, the Zn 1-x  Mg x  S y  Se 1-y  layer is used as an etching stoppig layer. Thus, selective etching of the ZnSe, ZnTe, ZnSSe or Zn 1-a  Mg a  S b  Se 1-b  layer can be conducted with excellent controllability and reproducibility.

This is a division of application Ser. No. 08/224,574, filed Apr. 7,1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a selective etching method, an optoelectronicdevice and its fabricating method suitable for use in, for example, asemiconductor laser using a II-VI compound semiconductor.

2. Description of the Prior Art

In recent years, demand for semiconductor lasers capable of emittinglight of short wavelengths has increased for improving the recordingdensity of optical discs and resolution of laser printers, and vigorousstudies have been done with the aim of realizing it.

The present Applicant, after earnest studies to meet such demand,already proposed a semiconductor laser using ZnMgSSe compoundsemiconductors as materials of cladding layers and Other layers andcapable of emitting blue to green light (for example, Japanese PatentApplication No. Hei 4-229356).

The above-mentioned semiconductor laser using ZnMgSSe compoundsemiconductors, in general, additionally includes one or more materialssuch as ZnSe compound semiconductors other than ZnMgSSe compoundsemiconductors. When fabricating the semiconductor laser, selectiveetching may be needed to process the ZnSe compound semiconductorprovided on a ZnMgSSe compound semiconductor layer into a mesa-type orother type pattern.

A method for etching the ZnSe compound semiconductor layer ! is a wetetching method using a K₂ Cr₂ O₇ :H₂ SO₄ etchant. In the wet etchingmethod, however, the etching rate of the ZnMgSSe compound semiconductorlayer is faster than the etching rate of the ZnSe compound semiconductorlayer, which results in the underlying ZnMgSSe compound semiconductorlayer being deeply etched during etching of the ZnSe compoundsemiconductor layer thereon.

Means heretofore available for preventing this problem is a method Ofestimating etching time required for etching the ZnSe compoundsemiconductor layer on the basis of the etching rate of the ZnSecompound semiconductor to control the etching depth in view of theestimated etching time. This method, however, involves a problem fromthe viewpoint of controllability and reproducibility.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a selectiveetching method excellent in controllability and reproducibility ofselective etching of ZnSe, ZnTe, ZnSSe or Zn_(1-a) Mg_(a) S_(b) Se_(1-b)compound semiconductor.

Another object of the invention is to provide an optoelectronic devicefabricated by selective etching of ZnSe, ZnTe, ZnSSe or Zn_(1-a) Mg_(a)S_(b) Se_(1-b) compound semiconductor with excellent controllability andreproducibility and hence improved in manufacturing yield.

Still another object of the invention is to provide a method forfabricating an optoelectronic device which can fabricate anoptoelectronic device by selective etching of ZnSe, ZnTe, ZnSSe orZn_(1-a) Mg_(a) S_(b) Se_(1-b) compound semiconductor with excellentcontrollability and reproducibility and! hence can be improved inmanufacturing yield.

According to an aspect of the invention, there is provided a selectiveetching method comprising the steps of:

providing at least one layer to be etched made of a ZnSe, ZnTe, ZnSSe orZn_(1-a) Mg_(a) S_(b) Se_(1-b) (0<a<1, 0<b<1) compound semiconductor onan etching stopping layer made of a Zn_(1-x) Mg_(x) S_(y) Se_(1-y)(0<x<1, 0<y<1, a<x) compound semiconductor; and

selectively etching the at least one layer to be etched by a dry etchingmethod.

For example, a reactive ion etching (RIE) method is used as the dryetching method.

According to another aspect of the invention, there is provided anoptoelectronic device comprising:

a first layer made of a Zn_(1-x) Mg_(x) S_(y) Se_(1-y) (0<x<1, 0<y<1)compound semiconductor; and

at least one second layer provided on the first layer and made of aZnSe, ZnTe, ZnSSe or Zn_(1-a) Mg_(a) S_(b) Se_(1-b) (0<a<1, 0<b<1, a<x)compound semiconductor.

The optoelectronic device according to the invention may be any one oflight emitting devices such as semiconductor lasers and light emittingdiodes, electronic devices such as transistors, and composite devices ofsuch light emitting devices and electronic devices.

An example of optoelectronic devices according to the invention is asemiconductor laser in which the first layer is a carrier blocking layerand the second layer is a cladding layer.

Another example of optoelectronic devices according to the invention isa distributed feedback semiconductor laser in which the first layer isan optical waveguide layer and the second layer is one which makes adiffraction grating.

According to still another aspect of the invention, there is provided amethod for fabricating an optoelectronic device comprising the steps of:

providing at least one second layer made of a ZnSe, ZnTe, ZnSSe orZn_(1-a) Mg_(a) S_(b) Se_(1-b) (0<a<1, 0<b<1) compound semiconductor ona first layer made of a Zn_(1-x) Mg_(x) S_(y) Se_(1-y) (0<x<1, 0<y<1,a<x) compound semiconductor; and

selectively etching the at least one second layer by a dry etchingmethod.

For example, a reactive ion etching (RIE) method is used as the dryetching method.

According to the selective etching method provided by the presentinvention, in case of etching the layer to be etched made of a ZnSe,ZnTe, ZnSSe or Zn_(1-a) Mg_(a) S_(b) Se_(1-b) (0<a<1, 0<b<1) compoundsemiconductor by dry etching methods such as the RIE method, since theetching selectivity of the layer to be etched can be made sufficientlylarge with respect to the underlying etching stopping layer made of theZn_(1-x) Mg_(x) S_(y) Se_(1-y) (0<x<1, 0<y<1, a<x) compoundsemiconductor, the etching can automatically be stopped at a point oftime when the etching has reached to expose the etching stopping layer.Therefore, selective etching of the ZnSe, ZnTe, ZnSSe or Zn_(1-a) Mg_(a)S_(b) Se_(1-b) compound semiconductor can be performed with excellentcontrollability and reproducibility.

According to the optoelectronic device provided by the presentinvention, when the second layer made of a ZnSe, ZnTe, ZnSSe or Zn_(1-a)Mg_(a) S_(b) Se_(1-b) (0<a<1, 0<b<1) compound semiconductor on the firstlayer made of a Zn_(1-x) Mg_(x) S_(y) Se_(1-y) (0<x<1, 0<y<1, a<x) isetched by dry etching methods such as the RIE method, the underlyingfirst layer behaves as an etching stopping layer to enable selectiveetching of the second layer with excellent controllability andreproducibility. As a result, an accurate thickness as designed for thefirst layer is realized, and the optoelectronic device is improved inmanufacturing yield.

According to the method for fabricating an optoelectronic deviceprovided by the present invention, when the second layer made of a ZnSe,ZnTe, ZnSSe or Zn_(1-a) Mg_(a) S_(b) Se_(1-b) (0<a<1, 0<b<1) compoundsemiconductor on the first layer made of a Zn_(1-x) Mg_(x) S_(y)Se_(1-y) (0<x<1, 0<y<1, a<x) is etched by dry etching methods such asthe RIE method, the underlying first layer behaves as an etchingstopping layer to enable selective etching of the second layer withexcellent controllability and reproducibility. As a result, an accuratethickness as designed for the first layer is realized, and theoptoelectronic device is improved in manufacturing yield.

In particular when the optoelectronic device is a semiconductor laser inwhich the first layer is a carrier blocking layer and the second layeris a cladding layer, an accurate thickness as designed for the carrierblocking layer is realized. Also when the optoelectronic device is adistributed feedback semiconductor laser in which the first layer is anoptical waveguide layer and the second layer is one which forms adiffraction grating, an accurate thickness as designed for the opticalwaveguide layer is realized.

The above, and other, objects, features and advantage of the presentinvention will become readily apparent from the following detaileddescription thereof which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 are cross-sectional views for explaining a method forfabricating a semiconductor laser according to a first embodiment of theinvention;

FIGS. 4 to 6 are cross-sectional views for explaining a method forfabricating a semiconductor laser according to a second embodiment ofthe invention; and

FIG. 7 is a graph showing a result of measurement of dependency of theetching rate on Mg composition ratio when etching a Zn_(1-x) Mg_(x)S_(y) Se_(1-y) layer by the RIE method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventors prepared samples by epitaxially growing on a GaAssubstrate a ZnSe layer, a ZnTe layer, a ZnSSe layer (in lattice matchingwith the GaAs substrate) and a ZnMgSSe layer (with Mg composition ratiobeing 0.09 and in lattice matching with the GaAs substrate),respectively, and conducted an experiment of etching with the samples byusing the reactive ion etching (RIE) method. In the RIE a mixed gas ofSiCl₄ and He was used as an etching gas and the same etching conditionswere used.

The experiment revealed the etching rate of the ZnMgSSe layer with thecomposition ratio of Mg being 0.09 to be about a half the etching rateof the ZnSe, ZnTe or ZnSSe layer.

Apart from the foregoing experiment, the present inventors preparedsamples by epitaxially growing on a GaAs substrate Zn_(1-x) Mg_(x) S_(y)Se_(1-y) layers different in Mg composition ratio and conducted anotherexperiment to know dependency of the etching rate of the Zn_(1-x) Mg_(x)S_(y) Se_(1-y) layer upon Mg composition ratio, and resulted as shown inFIG. 7 In these samples the Zn_(1-x) Mg_(x) S_(y) Se_(1'1) y layers werelattice-matched with the GaAs substrate.

Etching in this experiment, like the former experiment, was performed bythe RIE method using the mixed gas of SiCl₄ and He (SiCl₄ :He=10:8.6) asan etching gas. Power used was 100 W, and pressure was 3.3 pa (25mTorr). It is known from FIG. 7 that the etching rate the Zn_(1-x)Mg_(x) S_(y) Se_(1-y) layer exhibits a substantially linear decrease asthe Mg composition ratio x increases and that the etching rate decreasesby 10 nm/min when the Mg composition ratio x increases by 0.1 (10%).

Results of the experiments reveal that, during etching of the ZnSe, ZnTeOr ZnSSe layer provided on the ZnMgSSe layer by the RIE method, theZnMgSSe layer thereunder behaves as an etching stopping layer, andprovides excellent controllability and reproducibility in selectiveetching of the ZnSe, ZnTe or ZnSSe layer to be etched. Also when anotherZnMgSSe layer formed on the ZnMgSSe layer is etched by the RIE method,the underlying ZnMgSSe layer behaves as an etching stopping layer,provided the Mg composition ratio of the ZnMgSSe layer to be etched issufficiently smaller than that of the underlying ZnMgSSe layer, andenables selective etching of the ZnMgSSe layer to be etched withexcellent controllability and reproducibility.

The same applies not only when the RIE method is used but also whenother dry etching methods are used.

Embodiments of the invention are now described below with reference tothe drawings. In all the drawings illustrating the embodiments, commonreference numerals are assigned to common or equivalent elements.

FIGS. 1 to 13 are cross-sectional views showing sequential steps of amethod for fabricating a semiconductor laser according to a firstembodiment of the invention.

First, as shown in FIG. 1, the first embodiment epitaxially grows, forexample, on a (100)-oriented n-type GaAs substrate 1, an n-type ZnSebuffer layer 2, an n-type Zn_(1-x1) Mg_(x1) S_(y1) Se_(1-y1) claddinglayer 3, an active layer 4 made of, for example, ZnSe or ZnSSe layer, ap-type Zn_(1-x2) Mg_(x2) S_(y2) Se_(1-y2) layer 5 which is used as anetching stopping layer upon etching by the RIE method referred to laterand a carrier blocking layer for effectively accomplishing carrierconfinement, a p-type Zn_(1-x3) Mg_(x3) Mg_(x3) S_(y3) Se_(1-y3)cladding layer 6, and a p-type ZnSe cap layer 7 in sequence by, forexample, the molecular beam epitaxy (MBE) method, and thereafter makes astripe-shaped mask 8 on the p-type ZnSe cap layer 7. The mask 8 is madeof a material resistant to etching by the RIE method referred to later.In this case, the Mg composition ratio xi of the n-type Zn_(1-x1)Mg_(x1) S_(y1) Se_(1-y1) cladding layer 3 and the Mg composition ratiox3 of the p-type Zn_(1-x3) Mg_(x3) S_(y3) Se_(1-y3) cladding layer 6 maybe, for example, 0.1 (10%), and the Mg composition ratio x2 of thep-type Zn_(1-x2) Mg_(x2) S_(y2) Se_(1-y2) layer 5 may be, for example,0.25 (25%).

Next, by using the mask 8 and the RIE method using as an etching gas amixed gas of, for example, SiCl₄ and He, the method sequentially etchesthe p-type ZnSe cap layer 7 and the p-type Zn_(1-x3) Mg_(x3) S_(y3)Se_(1-y3) cladding layer 6 in the vertical direction of the substratesurface. As a result, as shown in FIG. 2, the p-type ZnSe cap layer 7and the p-type Zn_(1-x3) Mg_(x3) S_(y3) Se_(1-y3) cladding layer 6 areetched into a mesa-type pattern. Since the etching rate of theunderlying p-type Zn_(1-x2) Mg_(x2) S_(y2) Se_(1-y2) layer 5 having theMg composition ratio x2=0.25 is sufficiently smaller than the etchingrates of the p-type ZnSe cap layer 7 and the p-type Zn_(1-x3) Mg_(x3)S_(y3) Se₁₋₃ cladding layer 6 having the Mg composition ratio x3=0.1which are to be etched, the p-type Zn_(1-x2) Mg_(x2) S_(y2) Se_(1-y2)layer 5 behaves as an etching stopping layer during the etching process.Therefore, when the etching has reached to expose the p-type Zn_(1-x2)Mg_(x2) S_(y2) Se_(1-y2) layer 5, it is automatically stopped.

Next, after the mask 8 is removed, as shown in FIG. 3, a ZnSe layer 9 ofhigh resistance, for example, is epitaxially grown to fill oppositesides of the p-type Zn_(1-x3) Mg_(x3) S_(y3) Se_(1-y3) cladding layer 6and the p-type ZnSe cap layer 7 made into the mesa-type pattern.

Next, although not shown, a p-side electrode such as, for example, Auelectrode, in ohmic contact with the p-type ZnSe cap layer 7 is formed,and an n-side electrode such as, for example, In electrode, in ohmiccontact with the n-type GaAs substrate 1 is formed on the back surfaceof the n-type GaAs substrate 1.

After that, the n-type GaAs substrate 1 having the laser structure madethereon as explained above is cleaved into bars to! form reflective endsurfaces, and the bars are processed into chips. Then by packaging thechips, the target semiconductor lasers are finished.

As explained above, according to the first embodiment, since theunderlying p-type Zn_(1-x2) Mg_(x2) S_(y2) Se_(1-y2) layer 5 behaves asan etching stopping layer when the p-type ZnSe cap layer 7 and thep-type Zn_(1-x3) Mg_(x3) S_(y3) Se_(1-y3) cladding layer 6 are etched bythe RIE method, the p-type Zn_(1-x2) Mg_(x2) S_(y2) Se_(1-y2) layer 5 iseffectively prevented from being etched. As a result, as compared withthe existing method using a wet etching method and involving adifficulty in obtaining an accurate thickness as designed for the p-typeZn_(1-x2) Mg_(x2) Mg_(x2) S_(y2) Se_(1-y2) layer 5 because it controlledthe etching depth on the basis of the etching time, the invention canmake an accurate thickness as designed for the p-type Zn_(1-x2) Mg_(x2)S_(y2) Se_(1-y2) layer 5 which is used as a carrier blocking layer inthe final structure. Thus the semiconductor laser can be improved inmanufacturing yield.

FIGS. 4 to 6 are cross-sectional views showing sequential steps of amethod for fabricating a semiconductor laser according to a secondembodiment of the invention. The semiconductor laser according to thesecond embodiment is a distributed feedback semiconductor laser.

First, as shown in FIG. 4, by using, for example, the MBE method, thesecond embodiment epitaxially grows on, for example, a (100)-orientedn-type GaAs substrate 1, an n-type ZnSe buffer layer 2, an n-typeZn_(1-x1) Mg_(x1) S_(y1) Se_(1-y1) cladding layer 3 an active layer 4made of, for example, ZnSe or ZnSSe layer, a p-type Zn_(1-x2) Mg_(x2)S_(y2) Se_(1-y2) layer 5 which is used as an etching stopping layer uponetching by the RIE method referred to later and an optical! waveguidelayer, and a ZnSe layer 10 for forming a diffraction grating insequence, and thereafter makes on the p-type ZnSe layer 10 a mask 11having a pattern corresponding to the diffraction grating to be formed.The mask 11 is made of a material resistant to etching by the RIE methodreferred to later. The Mg composition ratio x2 of the p-type Zn_(1-x2)Mg_(x2) S_(y2) Se_(1-y2) layer 5 may be, for example, 0.10 (10%).

Next, by using the mask 11 and the RIE method using as an etching gas amixed gas of, for example, SiCl₄ and He, the method etches the ZnSelayer 10 in the vertical direction of the substrate surface. As aresult, as shown in FIG. 5, She ZnSe layer 10 is etched in a pattern ofthe diffraction grating desired. Since the etching rate of theunderlying p-type Zn_(1-x2) Mg_(x2) S_(y2) Se_(1-y2) layer 5 issufficiently smaller than the etching rate of the ZnSe layer 10 to beetched, the p-type Zn_(1-x2) Mg_(x2) S_(y2) Se_(1-y2) layer 5 behaves asan etching stopping layer during the etching process. Therefore, whenthe etching has reached So expose the p-type Zn_(1-x2) Mg_(x2) S_(y2)Se_(1-y2) layer 5, it is automatically stopped.

Next, as shown in FIG. 6, the mask 11 is removed, and after a p-typeZn_(1-x3) Mg_(x3) S_(y3) Se_(1-y3) cladding layer 6 is epitaxially grownto cover the diffraction grating 12 thus made, a p-type ZnSe cap layer 7is epitaxially grown thereon.

After that, in the same manner as the first embodiment, through !provision of a p-side electrode and an n-side electrode, cleavage, andother steps, the target distributed feedback semiconductor lasers arefinished.

According to the second embodiment, since the underlying p-typeZn_(1-x2) Mg_(x2) S_(y2) Se_(1-y2) layer 5 behaves as an etchingstopping=layer when the ZnSe layer 10 for making a diffraction gratingthereon is etched by the RIE method, the p-type Zn_(1-x2) Mg_(x2) S_(y2)Se_(1-y2) layer 5 is effectively prevented from being etched. As aresult, while making an accurate thickness as designed for the p-typeZn_(1-x2) Mg_(x2) S_(y2) Se_(1-y2) layer 5 which is used as an opticalwaveguide layer in the final structure, the diffraction grating 12 canbe made. Thus the distributed feedback semiconductor laser can beimproved in manufacturing yield.

Having described specific preferred embodiments of the present inventionwith reference to the accompanying drawings, it is to be understood thatthe invention is not limited to those precise embodiments, and thatvarious changes and modifications may be effected therein by one skilledin the art without departing from the scope or the spirit of theinvention as defined in the appended claims.

For example, although the first and second embodiments have beendescribed as using a mixed gas of SiCl₄ and He as the RIE etching gas, adifferent mixed gas of, for example, BCl₃ and He may be used as theetching gas.

The ZnSe layer 9 used as a filler layer in the first embodiment may alsobe replaced by, for example, a ZnSSe layer, a ZnMgSSe layer or otherinsulating materials such as polyimide, Al₂ O₃, etc.

The ZnSe layer 10 used for making the diffraction grating in the secondembodiment may also be replaced by a ZnSSe layer.

Further, the p-type ZnSe cap layer 7 used in the first and secondembodiments may be replaced by a p-type ZnTe cap layer, and a p-typeZnTe cap layer may be further provided on the p-type ZnSe cap layer 7.

Moreover, the particular values shown for the Mg composition ratios x1,x2 and x3 of the n-type Zn_(1-x1) Mg_(x1) S_(y1) Se_(1-y1) claddinglayer 3, the p-type Zn_(1-x2) Mg_(x2) S_(y2) Se_(1-y2) optical waveguidelayer 5 and the p-type Zn_(1-x3) Mg_(x3) S_(y3) Se_(1-y3) cladding layer6 are merely examples, and desired values may be chosen for thesecomposition ratios x1, x2 and x3.

As described above, according to the selective etching method providedby the invention, selective etching of ZnSe, ZnTe, ZnSSe or Zn_(1-a)Mg_(a) S_(b) Se_(1-b) compound semiconductor can be conducted withexcellent controllability and reproducibility.

According to the optoelectronic device and its fabricating methodprovided by the invention, because of its ZnSe, ZnTe, ZnSSe or Zn_(1-a)Mg_(a) S_(b) Se_(1-b) compound semiconductor being selective etched withexcellent controllability and reproducibility, the manufacturing yieldof the optoelectronic device can be improved.

What is claimed is:
 1. A selective etching method comprising the stepsof:providing at least one layer to be etched made of a ZnSe, ZnTe, ZnSSeor Zn_(1-a) Mg_(a) S_(b) Se_(1-b) (0<a<1, <b<1) compound semiconductoron an etching stopping layer made of a Zn_(1-x) Mg_(x) S_(y) Se_(1-y)(0<x<1, 0<y<1, a<x) compound semiconductor; and selectively etching saidat least one layer to be etched by a dry etching method.
 2. The methodaccording to claim 1 wherein said dry etching method is a reactive ionetching method.
 3. A method for fabricating an optoelectronic devicecomprising the steps of:providing at least one second layer made of aZnSe, ZnTe, ZnSSe or Zn_(1-a) Mg_(a) S_(b) Se_(1-b) (0<a<1, 0<b<1)compound semiconductor on a first layer made of a Zn_(1-x) Mg_(x) S_(y)Se_(1-y) (0<x<1, 0<y<1, a<x) compound semiconductor; and selectivelyetching said at least one second layer by a dry etching method.
 4. Themethod according to claim 3 wherein said dry etching method is areactive ion etching method.
 5. The method according to claim 3 whereinsaid optoelectronic device is a semiconductor laser, said first layerbeing a carrier blocking layer and said second layer being a claddinglayers.
 6. The method according to claim 3 wherein said optoelectronicdevice is a distributed feedback semiconductor laser, said first layerbeing an optical waveguide layer and said second layer being a layer forforming a diffraction grating.